Electrostatic discharge power clamp with fail-safe design

ABSTRACT

Electrostatic discharge protection circuits and methods of fabricating an electrostatic discharge protection circuit, as well as methods of protecting an integrated circuit from a transient electrostatic discharge event. The electrostatic discharge protection circuit includes a power clamp device, a first timing circuit with a first resistor and a first capacitor that is coupled with the first resistor at a first node, and a second timing circuit including a second resistor and a second capacitor that is coupled with the second resistor at a second node. The electrostatic discharge protection circuit further includes a logic gate with a first input coupled with the first node, a second input coupled with the second node, and an output coupled with the power clamp device. The logic gate responds to voltages at the first and second nodes to control the impedance state of the power clamp device.

BACKGROUND

The invention generally relates to semiconductor manufacturing andintegrated circuits and, more particularly, to electrostatic dischargeprotection circuits and methods of fabricating an electrostaticdischarge protection circuit, as well as methods of protecting anintegrated circuit from electrostatic discharge.

An integrated circuit may be exposed to transient electrostaticdischarge (ESD) events that can direct potentially large and damagingESD currents to the integrated circuits of the chip. An ESD eventinvolves an electrical discharge from a source, such as the human bodyor a metallic object, over a short duration and can deliver a largeamount of current to the integrated circuit. An integrated circuit maybe protected from ESD events by, for example, incorporating an ESDprotection circuit into the chip. If an ESD event occurs, the ESDprotection circuit triggers a power clamp device, such as asilicon-controlled rectifier, to enter a low-impedance, conductive statethat directs the ESD current to ground and away from the integratedcircuit. The ESD protection device holds the power clamp device in itsconductive state until the ESD current is drained and the ESD voltage isdischarged to an acceptable level.

Improved electrostatic discharge protection circuits that provideelectrostatic discharge protection and methods of fabricating anelectrostatic discharge protection circuit, as well as improved methodsof protecting an integrated circuit from a transient electrostaticdischarge event, are needed.

SUMMARY

In an embodiment of the invention, an electrostatic discharge protectioncircuit includes a power clamp device, a first timing circuit with afirst resistor and a first capacitor that is coupled with the firstresistor at a first node, and a second timing circuit including a secondresistor and a second capacitor that is coupled with the second resistorat a second node. The electrostatic discharge protection circuit furtherincludes a logic gate with a first input coupled with the first node, asecond input coupled with the second node, and an output coupled withthe power clamp device.

In an embodiment of the invention, a method is provided for fabricatingan electrostatic discharge protection circuit for a chip. The methodincludes forming, using a substrate, a first resistor and a firstcapacitor of a first timing circuit, and forming, using the substrate, asecond resistor and a second capacitor of a second timing circuit. Apower clamp device is formed using the substrate. The method furtherincludes forming, using the substrate, a logic gate including a firstinput coupled with a first node coupling the first capacitor with thefirst resistor, a second input coupled with a second node between thesecond capacitor and the second resistor, and an output coupled with thepower clamp device.

In another embodiment of the invention, a method is provided foroperating an electrostatic discharge protection circuit when power isapplied to a chip and the electrostatic discharge protection circuit onthe chip. The method includes supplying a first voltage to a first inputof a logic gate from a first node between a first resistor and a firstcapacitor of a first timing circuit, and supplying a second voltage to asecond input of the logic gate from a second node between a secondresistor and a second capacitor of a second timing circuit. The methodfurther includes outputting a third voltage from the logic gate to apower clamp device based on the first voltage and the second voltage.The third voltage places the power clamp device in a high-impedancestate.

In another embodiment of the invention, an electrostatic dischargeprotection circuit includes a power clamp device, a timing circuit, afirst field-effect transistor, and a second field-effect transistor. Thetiming circuit includes a resistor, a first capacitor that is coupledwith the resistor at a node, and a second capacitor that is coupled withthe resistor at the node. The field-effect transistor includes a firstgate and is coupled in series with the first capacitor between apositive rail of a power supply and a negative rail of the power supply.The electrostatic discharge protection circuit further includes adecoder with an output line coupled with gate of the field-effecttransistor. The decoder is configured to selectively output a voltage onthe output line to the gate of the field-effect transistor.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIG. 1 is a circuit diagram for a timing circuit in accordance with anembodiment of the invention.

FIG. 2 is a cross-sectional view of a correlated pair comprising aresistor and a capacitor of the timing circuit.

FIG. 3 is a circuit diagram for a timing circuit in accordance with anembodiment of the invention.

FIG. 4 is a circuit diagram for a timing circuit in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1, 2 and in accordance with an embodiment of theinvention, an electrostatic discharge (ESD) protection circuit 10 for achip generally includes a plurality of timing circuits 12, 13 that arearranged in branches, a driving circuit 14, a NOR gate 40, and a powerclamp device 16 coupled by the driving circuit 14 with the timingcircuit 12. The timing circuits 12, 13 are coupled between a positive(V_(DD)) rail 18 of a power supply and a negative (V_(SS)) rail 20 ofthe power supply. The V_(DD) rail 18 may be connected with a V_(DD)power pin and the V_(SS) rail 20 may be connected with a V_(SS) powerpin. Internal circuits 22 of a chip, which are protected by the ESDprotection circuit 10 on the chip, are also connected with the V_(DD)rail 18 and V_(SS) rail 20. The timing circuits 12, 13, the drivingcircuit 14, the NOR gate 40, and the power clamp device 16 may belocated on the chip.

The timing circuits 12, 13 are coupled in parallel between the V_(DD)rail 18 and the V_(SS) rail 20. The timing circuit 12 includes aresistor 28 and a capacitor 32 that are coupled in series between theV_(DD) rail 18 and the V_(SS) rail 20 with the resistor 28 coupled tothe capacitor 32 at a node 36. The timing circuit 13 includes a resistor30 and a capacitor 34 that are also coupled in series between the V_(DD)rail 18 and the V_(SS) rail 20 with the resistor 30 coupled to thecapacitor 34 at a node 38. Additional timing circuits like timingcircuits 12, 13 may be provided with correlated pairs of resistors andcapacitors coupled in series between the V_(DD) rail 18 and the V_(SS)rail 20 to provide additional redundancy.

The driving circuit 14 of the ESD protection circuit 10 includes a NORgate 40 and a plurality of inverters 42, 44 that couple the NOR gate 40with the power clamp device 16. The NOR gate 40, which is comprised ofp-channel transistors and/or n-channel transistors, includes a pluralityof inputs that may be equal to the number of correlated pairs ofresistors 28, 30 and capacitors 32, 34, which in turn is equal to thenumber of nodes 36, 38 and an output that is coupled with an input ofthe inverter 42. The NOR gate 40 is a digital logic gate that implementsa logical NOR truth table with Boolean logic applied to input logic inorder to generate output logic. If all of the inputs to the NOR gate 40from the nodes 36, 38 are at a voltage equal to logic 1 (i.e., high orV_(DD)), the voltage for the output logic signal is equal to logic 0(i.e., low or V_(SS)). If at least one of the inputs to the NOR gate 40from the nodes 36, 38 is at a voltage equal to logic 1, the voltage forthe output logic signal is equal to logic 0. If all of the inputs to theNOR gate 40 from the nodes 36, 38 are biased at a voltage equal to logic0, the voltage for the logic signal output by the NOR gate 40 is equalto logic 1.

The inverter 44 has an input that is coupled with an output from theinverter 42 and an output that is coupled at a node 46 with the powerclamp device 16. Each of the inverters 42, 44 is a digital logic gatethat implements a logical negation truth table with Boolean logicapplied to input logic in order to generate output logic. If the inputto either of the inverters 42, 44 is equal to a voltage equal to logic 1(i.e., high or VDD), the voltage for the respective output logic signalis equal to logic 0 (i.e., low or VSS). If the input to either of theinverters 42, 44 is equal to a voltage equal to logic 0, the voltage forthe respective output logic signal is equal to logic 1.

Generally, the driving circuit 14 includes one or more inverters 42, 44and features a two-stage configuration in the representative embodiment.However, the number of inverters 42, 44 may differ from therepresentative two-stage configuration in FIG. 1. For example, thenumber of inverters 42, 44 may comprise a four-stage configuration inorder to output the correct logic in the representative embodiment.While the NOR gate 40 is indicated as part of the driving circuit 14,the NOR gate 40 may be considered to be distinct from the drivingcircuit in some embodiments.

When the chip is unpowered and in response to a transient ESD event, thepower clamp device 16 may be triggered to switch from its high-impedancestate to its low-impedance state by the operation of the timing circuits12, 13 as orchestrated by the NOR gate 40. In its low-impedance state,the power clamp device 16 provides a current path to the V_(SS) rail 20with a current-carrying capacity that is sufficient to dissipate thelarge current produced by a transient ESD event at the V_(DD) power pinor the V_(SS) rail power pin.

The power clamp device 16 may be a metal-oxide-semiconductor transistorof large dimensions (e.g., a BigFET) having a gate with a width greaterthan one thousand microns that is coupled with the output from thedriving circuit 14, and may be constructed as either a p-channelfield-effect transistor or an n-channel field-effect transistor. In therepresentative embodiment, the power clamp device 16 is an n-channelfield-effect transistor. In alternative embodiments, the power clampdevice 16 may comprise a silicon controlled rectifier or a bipolarjunction transistor.

The resistors 28, 30 of the timing circuit 12 each have a discreteresistance value and, in a representative embodiment, may be comprisedof a polysilicon film resistor 48 (FIG. 2) that is formed by patterninga layer of polysilicon. The resistance of the polysilicon film resistor48 is based on its dimensions and the resistivity of the polysilicon. Inalternative embodiments, the resistors 28, 30 may comprise diffusionresistors, well resistors, etc.

The capacitors 32, 34 of the timing circuit 12 each have a discretecapacitance value. In the representative embodiment, each of thecapacitors 32, 34 may be comprised of one or more deep trench capacitors50. Each deep trench capacitor 50 includes capacitor plates (i.e.,electrodes) and an intervening dielectric layer formed using a deeptrench. In particular, each deep trench capacitor may have aconstruction as shown by the representative deep trench capacitor 50shown in FIG. 2. Deep trench capacitor 50 is formed by patterning asubstrate 52 with, for example, lithography, mask opening, and reactiveion etching to form a deep trench. After the deep trench is formed, adoped region 54 may be formed in the substrate by introducing a suitablep-type or n-type dopant using, for example, ion implantation. The dopedregion 54 supplies a common lower capacitor plate for the deep trenchcapacitor 50. A dielectric layer 56 (e.g., silicon dioxide, siliconoxynitride, silicon nitride, and/or hafnium oxide) is formed on thebottom and sidewall surfaces of the deep trench. The deep trench isfilled with a low resistivity material (e.g., copper, tungsten, titaniumnitride, and/or doped polysilicon) to supply an upper capacitor plate 58of the deep trench capacitor 50. Deep trench capacitors 50, which may befabricated in an array, are compact structures compared with other typesof capacitor structures that may be used in ESD protection timingcircuits. The deep capacitor 50 may be coupled with the resistor 48 bywiring 49 to define a node that represents one or the other of the nodes36, 38.

In an embodiment, each of the capacitors 32, 34 may include only asingle deep trench capacitor like deep trench capacitor 50. In anotherembodiment, each of the capacitors 32, 34 may include an array or a bankof deep trench capacitors like deep trench capacitor 50 that are wiredtogether in parallel. In alternative embodiments, the capacitors 32, 34may comprise metal-insulator-metal capacitors, metal-oxide-semiconductorcapacitors, etc.

In the representative embodiment, the power clamp device 16, the NORgate 40, and the inverters 42, 44 (as well as other devices describedherein that are constructed from transistors) of the ESD protectioncircuit 10 may be comprised of n-channel or p-channel field-effecttransistors that are fabricated by complementarymetal-oxide-semiconductor (CMOS) processes. For example, each of theinverters 42, 44 includes a p-channel field-effect transistor and ann-channel field-effect transistor coupled in series with the p-channelfield-effect between the V_(DD) rail 18 and the V_(SS) rail 20. Each ofthe field-effect transistors in the ESD protection circuit 10 mayinclude a gate electrode, a gate dielectric layer positioned between thegate electrode and a semiconductor layer, and source/drain regions inthe semiconductor layer. The conductor constituting the gate electrodemay comprise, for example, metal, silicide, polycrystalline silicon(polysilicon), or any other appropriate material(s) deposited by achemical vapor deposition process, etc. The gate dielectric may becomprised of a layer of a dielectric or insulating material such assilicon dioxide, silicon oxynitride, hafnium oxide, etc. Thesource/drain regions may be formed by selectively doping thesemiconductor layer with ion implantation, dopant diffusion, etc.Middle-of line and back-end-of-line (BEOL) processing ensues to providean interconnect structure with wiring for power and signal transmission.In particular, the wiring of the interconnect structure may coupletogether the different device structures as diagrammatically shown inFIG. 1 (and other drawing views herein).

In use and with the chip unpowered, the ESD protection circuit 10 mayrespond to a transient ESD event that applies an ESD potential betweenthe V_(DD) rail 18 and the V_(SS) rail 20. The response time of the ESDprotection circuit 10 may be governed by the shorter of a time constantcharacterizing the timing circuit 12 and a different time constantcharacterizing the timing circuit 13. The time constant of timingcircuit 12 is based on a product of the electrical resistance ofresistor 28 and capacitor 32. The time constant of timing circuit 13 isbased on a product of the electrical resistance of resistor 28 andcapacitor 32, 34. In an embodiment, the electrical resistance of each ofthe resistors 28, 30 is equal and the capacitance of each of thecapacitors 32, 34 is equal so that the timing circuits 12, 13 have equaltime constants. Regardless of whether the capacitors 32, 34 arefunctional or non-functional, each of the timing circuits 12, 13 willoutput a voltage capable of triggering the power clamp device 16 inresponse to a transient ESD event at the V_(DD) power pin or the V_(SS)rail power pin. The NOR gate 40 will output a voltage equal to highbecause all of the inputs to the NOR gate 40 are low. The drivingcircuit 16 will subsequently transfer the voltage of V_(DD) from theoutput of the NOR gate 40 to the node 46, which will switch on the powerclamp device 16 to provide its low-impedance state. In its low-impedancestate, the power clamp device 16 defines a low-impedance current path toground at the V_(SS) rail 20 such that the ESD current is safelydiverted away from the internal circuits 22. After the current from thetransient ESD event dissipates, the power clamp device 16 returns to itshigh-impedance state as the voltage at the node 46 is removed.

In use and when the chip is powered on using the power supply, the ESDprotection circuit 10 provides fail-safe operation. If the capacitors32, 34 are functional and non-defective, both of the inputs to the NORgate 40 will be equal to logic 1 (i.e., high or V_(DD)) when the chip isinitially powered. The output from the NOR gate 40 will be equal tologic 0 (i.e., low or V_(SS)), which is then applied as thecorresponding voltage of V_(SS) at the node 46 to the power clamp device16. In the representative embodiment, the low voltage at the node 46will maintain the power clamp device 16 in its high-impedance state thatisolates the V_(DD) rail 18 from the V_(SS) rail 20 while the chip ispowered by the power supply.

One or more of the capacitors 32, 34 may be fabricated in a defectivecondition or may become defective during use such that one or more ofthe capacitors 32, 34 exhibits an abnormally-low impedance (i.e.,shorted to ground relative to the respective resistor). When the chip isnot powered, a defective capacitor will have a minimal effect on theperformance of the ESD protection circuit 10 as the branch of the timingcircuit 12 containing the defective capacitor has an infinite timeconstant. The timing circuit 12 will continue to trigger the power clampdevice 16 to furnish ESD protection for the unpowered chip.

When an attempt is made to initially power the chip, the ESD protectioncircuit 10 is configured to react to any of the capacitors 32, 34 beingin a defective condition. In this situation, the ESD protection circuit10 is configured to maintain the power clamp device 16 in itshigh-impedance state and to not allow the defective capacitor to causethe power clamp device 16 to be placed in its low-impedance state sothat a large current is directed through the power clamp device 16 toground. In an embodiment in which the power clamp device 16 is a BigFETwith a gate length greater than or equal to one thousand microns, theunwanted large current that is averted by the ESD protection circuit 10may amount to several amperes.

To permit the chip to be successfully powered on using the power supply,the ESD protection circuit 10 is configured to provide a fail-safedesign that responds to one or the other of the capacitors 32, 34 beingin a defective condition. Specifically, if at least one but fewer thanall of the capacitors 32, 34 are in a defective condition, the NOR gate40 causes a voltage equal to logic 0 (i.e., low or V_(SS)) to be appliedto the power clamp device 16 so that the power clamp device 16 is placedin its high-impedance state. The ESD protection circuit 10 prevents thenode 46 feeding the power clamp device 16 from being pulled high due tothe presence of a defective capacitor and, thereby, presents the V_(DD)rail 18 from being directly shorted to the V_(SS) rail 20 through theturned-on power clamp device 16.

As an example, if the voltage at node 36 is equal to logic 0 because ofa defective capacitor 32 and the voltage at node 38 is equal to logic 1because of a non-defective (i.e., functional) capacitor 34, the inputsto the NOR gate 40 will be equal to logic 1 and logic 0. The output fromthe NOR gate 40 will be equal to logic 0, which is then applied as thecorresponding voltage of V_(SS) at the node 46 to the power clamp device16. While the chip is powered, the low voltage at the node 46 will bemaintained and the power clamp device 16 will be maintained in itshigh-impedance state so the V_(DD) rail 18 is electrically isolated fromthe V_(SS) rail 20.

The NOR gate 40 outputs a voltage equal to logic 1 (i.e., high orV_(DD)) only if all of the inputs to the NOR gate 40 from the nodes 36,38 are equal to logic 0. This represents a condition in which all of thecapacitors 32, 34 are defective. Increasing the number of timingcircuits 12, 13 operates to decrease the probability that the NOR gate40 will output a voltage equal to logic 1 when the chip is powered. As aresult, the fail-safe nature of the design may be improved by increasingthe number of timing circuits 12, 13 and the corresponding number ofinputs to the NOR gate 40. If only one of the timing circuits 12, 13contains a functional capacitor, then the ESD protection circuit 10 willpermit the chip to be powered on. The redundancy present in the ESDprotection circuit 10 allows a larger number of defective capacitors tobe tolerated in comparison with conventional ESD protection circuitsthat lack such redundancy. Due to the redundancy in the timing circuits12, 13, the chip carrying the ESD protection circuit 10 is less likelyto be considered faulty during electrical testing and subsequentlyscrapped.

In an alternative embodiment in which the power clamp device 16 is ap-channel field-effect transistor, the number of inverters in thedriving circuit 14 may be modified to provide the correct control logicin response to the output from the NOR gate 40.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with an alternative embodiment, theNOR gate 40 may be replaced in the ESD protection circuit 10 by a NANDgate 60 and a plurality of inverters 62, 64 in order to form an ESDprotection circuit 61. In addition, the driving circuit 14 of the ESDprotection circuit 61 only includes the inverter 44, which couples theoutput of the NAND gate 60 with the power clamp device 16. Generally,the driving circuit 14 includes one or more inverters 44 and features aone-stage configuration in the representative embodiment. However, thedriving circuit 14 may include additional inverters to form, forexample, a three-stage configuration.

The NAND gate 60 is a digital logic gate, which is comprised oftransistors, that implements a logical conjunction truth table withBoolean logic applied to output a logic signal. The NAND gate 60includes inputs that are coupled, respectively, by the inverters 62, 64with the nodes 36, 38 of the timing circuit 12 and an output that iscoupled with the input to inverter 44. If any or all of the inputs tothe NAND gate 60 from the nodes 36, 38, as modified by the operation ofthe inverters 62, 64, supplies a voltage equal to logic 0 (i.e., low orV_(SS)), the voltage for the output logic signal is equal to logic 1(i.e., high or V_(DD)). The inverter 44 outputs a voltage representingthe opposite logic level to the input received from the NAND gate 60. Asa result, the inverter 44 outputs a voltage equal to logic 0 if theoutput received from the NAND gate 60 is equal to logic 1 so that thepower clamp device 16 is placed in its high-impedance state, and theinverter 44 outputs a voltage equal to logic 1 if the output from theNAND gate 60 is equal to logic 0 so that the power clamp device 16 isplaced in its low-impedance state.

The ESD protection circuit 61 functions similarly to ESD protectioncircuit 10 during a transient ESD event occurring at one or the other ofthe V_(DD) power pin or the V_(SS) rail power pin. The ESD protectioncircuit 61 will cause the power clamp device 16 to be placed in itslow-impedance state to divert the ESD current away from the internalcircuits 22.

The ESD protection circuit 61 also functions similarly to ESD protectioncircuit 10 when the chip is powered using the power supply. For example,if the capacitors 32, 34 are functional and the voltages at the node 36,38 are both equal to logic 1, the input through inverter 62 to the NANDgate 60 will be equal to logic 0 and the input through inverter 62 tothe NAND gate 60 will be equal to logic 0. The output from the NAND gate60 will be equal to logic 1, which is then inverted by inverter 44 andapplied as the corresponding voltage of V_(SS) at the node 46 to thepower clamp device 16. In the representative embodiment, the low voltageat the node 46 will maintain the power clamp device 16 in itshigh-impedance state that isolates the V_(DD) rail 18 from the V_(SS)rail 20 when the chip is powered by the power supply.

As another example, if the voltage at the node 36 is equal to logic 0because of a defective capacitor 32 and the voltage at the node 38 isequal to logic 1 because of a non-defective (i.e., functional) capacitor34, the input through inverter 62 to the NAND gate 60 will be equal tologic 1 and the input through inverter 64 to the NAND gate 60 will beequal to logic 0. The output from the NAND gate 60 will be equal tologic 1, which is then inverted to logic 0 by the inverter 44 andapplied as the corresponding voltage of V_(SS) at the node 46 to thepower clamp device 16 so that the power clamp device 16 is maintained inits high-impedance state.

The voltage for the logic signal output by the NAND gate 60 is equal tologic 0 (and inverted by inverter 44 to logic 1) only if all of theinputs to the NAND gate 60 from the nodes 36, 38 are equal to logic 1.This condition exists if both of the capacitors 32, 34 are defective. Asdiscussed above with respect to ESD protection circuit 61, increasingthe number of timing circuits 12, 13 to increase the redundancy mayincrease the tolerance to defective capacitors and contributes toincreasing the robustness of the fail-safe design.

In an alternative embodiment in which the power clamp device 16 is ap-channel field-effect transistor, the number of inverters in thedriving circuit 14 may be modified to provide the correct control logicin response to the output from the NAND gate 60.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with an alternative embodiment, anESD protection circuit 70 includes the capacitors 32, 34 while theresistor 24 includes only a single resistor that is shared in commonwith the capacitors 32, 34 in the timing circuit 12. The ESD protectioncircuit 70 includes a plurality of field-effect transistors 72, 74 and adecoder 76 coupled in parallel with the gate of each of the field-effecttransistors 72, 74. The source and drain of field-effect transistor 72are coupled in series with capacitor 32 between the node 36 and theV_(SS) rail 20. Similarly, the source and drain of field-effecttransistor 74 are coupled in series with capacitor 34 between the node38 and the V_(SS) rail 20. When the chip is unpowered, the ESDprotection circuit 70 operates as described hereinabove with respect toESD protection circuit 10 to respond to a transient ESD event.

The decoder 76 is a digital logic device represented by a combinationalcircuit that converts binary information received from address pins 78,80 on input lines 82, 84 to binary information output on output lines86, 88. The number of input lines 82, 84 may differ from the number ofoutput lines 86, 88. The decoder 76 may be comprised of a plurality offield-effect transistors wired to form one or more AND gates, one ormore NAND gates, etc. and coupled to provide the desired binaryinformation conversion.

When the chip is powered, the decoder 76 is addressable and programmablevia address pins 78, 80 to provide voltages to the gates of thefield-effect transistors 72, 74 for controlling the field-effecttransistors 72, 74. Specifically, in response to the input of voltagesconveying binary information via the input lines 82, 84 from the addresspins 78, 80, the decoder 76 can output control logic at voltages overoutput lines 86, 88 that permit the transistors 72, 74 to beindividually controlled and programmed Normally and under a condition inwhich the capacitors 32, 34 are functional, the output of the decoder 76biases the gates of the transistors 72, 74 so that all of thefield-effect transistors 72, 74 are switched to a low-impedance state.As a result, each of the capacitors 32, 34 is individually coupled in acurrent path with the V_(SS) rail 20 if the respective one of thefield-effect transistors 72, 74 is placed by the operation of thedecoder 76 in its low-impedance state. In one embodiment, thetransistors 72, 74 may be NMOSFETs and the pins 78, 80 are set so thatthe decoder 76 biases the gates of the transistors 72, 74 with a voltageequal to logic 1 that places the transistors 72, 74 in their respectivelow-impedance states.

The ESD protection circuit 70 may be configured to detect the powerclamp device 16 unexpectedly switching on at the time of power on anddraining a large amount of current. This type of incident at chip poweron may be the result of one or the other of the capacitors 32, 34 beingdefective and, as a consequence, appearing as a short to its respectiveresistor 28, 30. In response, the address pins 78, 80 of the ESDprotection circuit 70 are used to investigate the incident and topinpoint the capacitor that is the source of the short.

Specifically, the address pins 78, 80 are used to systematically turn oneach of the transistors 72, 74 while turning off all other transistorswith output voltages supplied through the output lines 86, 88. As eachof the transistors 72, 74 is individually switched to its low-impedancestate by the decoder 76 using the address pins 78, 80, the V_(DD)current is monitored for a large current flow indicative of a defectivecapacitor. In this manner, the defective capacitor can be identified andlogged. After full testing, the decoder 76 is programmed to switch thetransistors 72, 74 corresponding to defective capacitors to a voltagethat disables such defective capacitors. If one or more of thecapacitors 32, 34 are defective, the testing to provide the programmedstate permits the chip to be successfully powered on withoutexperiencing a short to ground through the defective capacitor. In anembodiment in which the transistors 72, 74 are n-channel field-effecttransistors, the decoder 76 is programmed to output a voltage equal tologic 0 (i.e., low or V_(SS)) to switch any of the transistors 72, 74that are in series with a defective capacitor to their high-impedancestate and any of the transistors 72, 74 that are in series with afunctional capacitor to their low-impedance state.

It will be understood that when an element is described as being“connected” or “coupled” to or with another element, it can be directlyconnected or coupled to the other element or, instead, one or moreintervening elements may be present. In contrast, when an element isdescribed as being “directly connected” or “directly coupled” to or withanother element, there are no intervening elements present. When anelement is described as being “indirectly connected” or “indirectlycoupled” to or with another element, there is at least one interveningelement present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. An electrostatic discharge protection circuitcomprising: a power clamp device; a first timing circuit including afirst resistor and a first capacitor that is coupled with the firstresistor at a first node; a second timing circuit including a secondresistor and a second capacitor that is coupled with the second resistorat a second node; and a logic gate including a first input coupled withthe first node, a second input coupled with the second node, and anoutput coupled with the power clamp device.
 2. The electrostaticdischarge protection circuit of claim 1 wherein the first capacitor andthe second capacitor are each comprised of one or more deep trenchcapacitors.
 3. The electrostatic discharge protection circuit of claim 1wherein the first timing circuit is coupled in series between a positiverail of a power supply and a negative rail of the power supply, and thesecond timing circuit is coupled in series between the positive rail ofthe power supply and the negative rail of the power supply.
 4. Theelectrostatic discharge protection circuit of claim 1 wherein the logicgate is a NOR gate.
 5. The electrostatic discharge protection circuit ofclaim 1 wherein the logic gate is a NAND gate, and further comprising: afirst inverter coupling the first node with the first input to the NANDgate; and a second inverter coupling the second node with the secondinput to the NAND gate.
 6. The electrostatic discharge protectioncircuit of claim 1 wherein the power clamp device is a field-effecttransistor including a gate with a width greater than one thousandmicrons.
 7. The electrostatic discharge protection circuit of claim 1further comprising: one or more inverters arranged to couple the logicgate with the power clamp device.
 8. The electrostatic dischargeprotection circuit of claim 1 wherein the first resistor and the secondresistor have equal electrical resistances, the first capacitor and thesecond capacitor have equal capacitances, and the first timing circuitand the second timing circuit have equal time constants.
 9. A method offabricating an electrostatic discharge protection circuit for a chip,the method comprising: forming, using a substrate, a first resistor anda first capacitor of a first timing circuit; forming, using thesubstrate, a second resistor and a second capacitor of a second timingcircuit; forming, using the substrate, a power clamp device; andforming, using the substrate, a logic gate including a first inputcoupled with a first node coupling the first capacitor with the firstresistor, a second input coupled with a second node between the secondcapacitor and the second resistor, and an output coupled with the powerclamp device.
 10. The method of claim 9 wherein the first capacitor isformed using a first deep trench defined in the substrate and the secondcapacitor is formed using a second deep trench defined in the substrate.11. The method of claim 9 further comprising: coupling the first timingcircuit in series between a positive rail of a power supply and anegative rail of the power supply; and coupling the second timingcircuit in series between the positive rail of the power supply and thenegative rail of the power supply.
 12. The method of claim 9 wherein thelogic gate is a NOR gate.
 13. The method of claim 9 wherein the logicgate is a NAND gate, and further comprising: forming, using thesubstrate, a first inverter coupling the first node with the first inputto the NAND gate; and forming, using the substrate, a second invertercoupling the second node with the second input to the NAND gate.
 14. Amethod of operating an electrostatic discharge protection circuit whenpower is applied to a chip and the electrostatic discharge protectioncircuit on the chip, the method comprising: supplying a first voltage toa first input of a logic gate from a first node between a first resistorand a first capacitor of a first timing circuit; supplying a secondvoltage to a second input of the logic gate from a second node between asecond resistor and a second capacitor of a second timing circuit; andoutputting a third voltage from the logic gate to a power clamp devicebased on the first voltage and the second voltage, wherein the thirdvoltage places the power clamp device in a high-impedance state.
 15. Themethod of claim 14 wherein the first capacitor is a non-defective deeptrench capacitor, the second capacitor is a defective deep trenchcapacitor, the first voltage is equal to a power supply voltage, and thesecond voltage is equal to ground.
 16. The method of claim 14 whereinthe logic gate is a NOR gate, and outputting the third voltage from theNOR gate to the power clamp device based on the first voltage and thesecond voltage comprises: determining an output from the NOR gate byapplying a logical NOR truth table to the first voltage and the secondvoltage.
 17. The method of claim 14 wherein the logic gate is a NANDgate, and outputting the third voltage from the NAND gate to the powerclamp device based on the first voltage and the second voltagecomprises: inverting the first voltage before the first voltage issupplied to the first input of the logic gate; inverting the secondvoltage before the second voltage is supplied to the second input of thelogic gate; determining an output from the NAND gate by applying alogical conjunction truth table to the inverted first voltage and theinverted second voltage.
 18. The method of claim 14 wherein the firstresistor and the second resistor have equal electrical resistances, thefirst capacitor and the second capacitor have equal capacitances, andthe first timing circuit and the second timing circuit have equal timeconstants.
 19. An electrostatic discharge protection circuit comprising:a power clamp device; a timing circuit including a resistor, a firstcapacitor that is coupled with the resistor at a node, and a secondcapacitor that is coupled with the resistor at the node; a firstfield-effect transistor including a first gate, the first field-effecttransistor coupled in series with the first capacitor between a positiverail of a power supply and a negative rail of the power supply; and adecoder including a first output line coupled with the first gate, thedecoder configured to selectively output a first voltage on the firstoutput line to the first gate.
 20. The electrostatic dischargeprotection circuit of claim 19 further comprising: a second field-effecttransistor including a second gate, the second field-effect transistorcoupled in series with the second capacitor between the positive rail ofthe power supply and the negative rail of the power supply, wherein thedecoder includes a second output line coupled with the second gate, thedecoder configured selectively output a second voltage on the secondoutput line to the second gate.